1. Field of the Invention
The present invention relates to an electrically conductive paste for use in fabricating electrodes having an ohmic contact on a semiconductor ceramic component such as a positive temperature coefficient (PTC) and the like, and to a semiconductor ceramic component using the same.
2. Background of the Invention
An electrode material capable of forming ohmic contacts have been utilized in various types of semiconductor ceramic elements using, for example, perovskite-type semiconductor ceramics such as barium titanate, zinc oxide-type semiconductor ceramics, or iron oxide-type semiconductor ceramics.
Electrode materials capable of forming an ohmic contact include an indium-gallium (In-Ga) alloy, a nickel film formed by electroless-plating, or an ohmic conductive paste. An ohmic conductive paste comprises a powder of a non-metallic element (e.g., zinc (Zn), aluminum (Al), or nickel (Ni)) as the electrically conductive component with a glass frit having a low melting point based on lead borosilicate or zinc borosilicate added therein.
More specifically, in case of using a conductive paste, an electrically conductive paste is prepared by dispersing a powder of a non-metallic element (e.g., Zn, Al, or Ni) and a powder of lead borosilicate glass frit or zinc borosilicate glass frit in an organic vehicle previously prepared by dissolving a binder resin in an organic solvent. Then, the electrically conductive paste thus obtained is applied to a semiconductor ceramic element, dried, and baked to remove organic components therefrom. Thus, an electrode having an ohmic contact (sometimes referred to simply hereinafter as "an ohmic contact electrode") and comprising an electrically conductive component and a glass frit is formed on the semiconductor element.
However, when a conventional ohmic contact electrode using a conductive paste is employed as an electrode of a semiconductor ceramic element such as a positive temperature coefficient (PTC) thermistor, for instance, the following problems have yet to be overcome.
A conventional ohmic contact electrode using a lead borosilicate glass frit as the glass component thereof is inferior in moisture resistance. Accordingly, a high resistance layer of an oxide and the like tends to form easily on the surface of the electrode. Thus, in case of a structure comprising a metallic contact pressure-welded to the electrode of a PTC thermistor, e.g., a PTC thermistor for a heater, a high resistance layer formed between the electrode and the metallic contact impairs the reliability of the PTC thermistor.
Furthermore, the electrodes obtained heretofore are porous. Thus, the resistance of the ohmic contact formed at the boundary between the PTC thermistor and the electrode tends to increase due to the absorbed moisture. This also lowers the reliability of the PTC thermistor.
Moreover, in the case when a glass frit based on zinc borosilicate is used as the glass component of the electrode, the glass frit undergoes reaction with a PTC thermistor element lowering the resistance of the element. This reduces the electrical-thermal conversion coefficient of a PTC thermistor when used in a heater.
Accordingly, an object of the present invention is to provide an electrically conductive paste which enables a moisture-resistant electrode capable of implementing a favorable ohmic contact when applied and baked on a semiconductor ceramic element. It is also an object of the present invention to provide a semiconductor ceramic component using the same.